The present invention relates to planetary mechanisms for coupling the rotary motion of a first shaft to a second shaft; more particularly, to planetary traction transmissions for rotational speed reduction; and most particularly, to a planetary traction drive mechanism for actuating a throttle valve shaft.
Planetary gear transmissions for coupling the rotation of a first shaft driving a sun gear to one or more planetary gears driving a second shaft are well known. Such transmissions inherently are rotation ratio reducers and, conversely, force multipliers wherein the rotation rate of the second shaft is always less than the rotation rate of the first shaft (ratio less than 1.0). It is further known to use traction rollers in place of the sun gear and planetary gears, although such systems require means for maintaining sufficient traction between the sun roller and the planetary rollers to prevent slippage therebetween when the torque load required by the application is applied to the transmission.
Traction may be increased by exerting inward radial force on the planetary rollers. For example, U.S. Pat. No. 771,541 to Ericson discloses a xe2x80x9cfriction gearxe2x80x9d system wherein the diameter of a driven split-ring surrounding the planets may be reduced by mechanically constricting the ring to compress the planets radially and thereby control slippage between the sun roller and the planetary rollers. This system requires continuous slippage monitoring and does not possess inherently high resistance to such slippage.
This system is also illustrative of a first type of planetary drive system wherein the axes of the planetary rollers are stationary and the rollers do not revolve about the sun gear; instead, the outer ring and driven shaft are urged by rotation of the rollers to rotate in a direction counter to the rotation direction of the input drive shaft. Such a mechanism is employed, for example, in marine windlasses and capstans.
For another example, U.S. Pat. No. 3,060,767 to Parrett discloses a speed changing mechanism wherein planetary rollers contain an intermediate layer of an elastomer such as neoprene and outer cylindrical members made of steel having sufficient elasticity to permit deflection. In assembling the mechanism, the planetary rollers are pre-stressed radially before insertion between the outer ring and the sun roller. Release of the planetary rollers after assembly causes high pressures to be exerted at the points of contact thereof with the outer ring and the sun roller xe2x80x9cto obtain high torque drive transmission therebetween without slipping.xe2x80x9d This system requires very high quality machining of the surface and diameter of the outer ring, the outer members of the planetary rollers, and the sun roller to provide a controlled interference assembly. During use of the device, radial forces, and hence slippage, may be affected by thermal expansion of the components.
This system is also illustrative of a second type of planetary drive system wherein the axes of the planetary rollers are mounted on a rotatable plate supporting an output shaft and the rollers revolve about the sun gear. The outer ring is stationary, and rotation and precession of the rollers about the sun roller cause the plate and output shaft to rotate in the same direction as the rotation direction of the input drive shaft.
Traction also may be increased by providing shear-thickening fluids to the roller surfaces. Such non-Newtonian fluids, when subjected to shear stress, undergo a viscosity increase, thereby increasing traction. A known use for such a system is as reduction gear for a naval vessel powered by a gas turbine engine. Such systems require costly and difficult high-quality machining of components, and the required exotic fluids are subject to decomposition and fatigue in use.
What is needed is a simple, inexpensive planetary traction drive mechanism wherein adequate traction results from a combination of radial compression of planetary rollers and the inherent tractive properties of the materials used to form the sun roller and planetary rollers.
Further, in the automotive arts, it is desirable to be able to electromechanically adjust precisely, rapidly, and reliably, the throttle-shaft angle of a throttle of an internal combustion engine. In the prior art, such adjustment typically is provided by an expensive torque or stepper motor or by a conventional electric motor and geared transmission. Such a transmission can include a pinion gear, a compound idler gear, and a throttle shaft driven gear incorporated into a gearbox on the side of the throttle body. Typically this gearbox is undesirably large. Further, the geartrain requires tight tolerance relationships between gear centerlines, requiring precision control of both gear and gearbox dimensions. Further, this arrangement offers no opportunity to isolate the gears from impact loads due to contact with minimum and maximum mechanical stops for shaft rotation, which adversely affects gear durability.
What is needed is a simple, compact, inexpensive electromechanical means for rotating a throttle shaft through a working angle of about 90xc2x0.
It is a principal object of this invention to provide an improved planetary traction drive mechanism wherein adequate traction results from a combination of radial compression of planetary rollers and inherent tractive properties of the materials used to form the sun roller and planetary rollers.
It is a further object of this invention to provide an improved throttle assembly for an internal combustion engine including a simple, compact, inexpensive electromechanical means for rotating the throttle shaft through a working angle of about 90xc2x0.
Briefly described, the present invention is directed to an improved planetary traction drive mechanism and to an improved throttle valve including the improved actuator mechanism.
The housing of an electric drive motor supports at one end a cup-shaped member open at an outer end and defining the outer ring of a planetary drive mechanism. The input drive shaft of the motor extends axially into the cup-shaped member to define the sun friction roller of a planetary traction drive system and an annular space between the sun roller and the outer ring. A circular output plate is rotatably disposed on the motor drive shaft, and one or more pins, preferably three pins equilaterally disposed, extend from the surface of the plate into the annular space, parallel to the drive shaft, and rotatably support one or more planetary friction rollers disposed in radial compression between the outer ring and the sun roller. On the surface of the output plate opposite the pins an output shaft extends generally parallel to the rotational axes of the sun roller and planetary rollers. The output shaft may be coaxially aligned with the sun roller, or with any of the planetary rollers, or with none of them, and may be connected via a link to a lever arm on a device to be rotated, for example, the throttle shaft of a throttle valve on an internal combustion engine.
The planetary rollers are formed from a resilient composite polymer which is mildly compressible, has a suitably high coefficient of sliding friction against metal, a low coefficient of wear, and a high resistance to flexural fatigue. The rollers are formed having unstressed diameters greater than the radial distance between the sun roller and the friction ring, so that the rollers must be compressed slightly in order to be installed therein, thereby creating tractive friction with the sun roller and the friction ring.
The rollers may be formed as solid disks of resilient polymer or as deformable cylinders of resilient polymer loosely positioned on the pins.